Angular Leaf Spot in Strawberries

Often considered a minor pathogen, angular leaf spot caused by the bacterium, Xanthomonas fragariae, can cause serious leaf and calyx infections ruining the marketability of fruit if left uncontrolled. Like all bacterium, the pathogen will infect leaves and the calyx through natural openings or wounds.

Infections can often start in production operations and come in on infected bare root transplants or cuttings. Symptoms on leaves include initial small, irregular water-soaked lesions (Fig. 1).

Fig. 2. Reddish-brown lesions on infected strawberry leaf caused by angular leaf spot. Note the translucent spots when held up to light. Photo by P. Nitzsche

As disease progresses lesions will enlarge and coalesce forming reddish-brown spots on leaf surfaces which later become necrotic and translucent (Fig. 2). Holding infected leaves up to the light will reveal this diagnostic feature.

Importantly, in severe outbreaks in the spring, the bacterium can spread to fruit causing the calyx to turn brown and dry out ruining the marketability of infected fruit (Figs. 3 & 4).

The pathogen is easily spread via overhead irrigation and during rain and can be moved around the field during harvest. The bacterium may overwinter on infected plant debris buried in the soil for up to one year. Conventional or organic copper-based products can help suppress the development of ALS, and should be applied at a low rate to avoid phytotoxicity in leaves. Weekly, preventative applications of 0.3 lb fixed copper have been shown to be effective in reducing ALS if applied early enough when disease pressure was still low.